A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning structure, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate, for example, for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” in such context herein may be considered as synonymous with the more general term “projection system”.
The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.
The term “patterning structure” used herein should be broadly interpreted as referring to a structure that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
A patterning structure may be transmissive or reflective. However, beneath a certain wavelength the use of a transmissive patterning structure is no longer possible due to the lack of suitable materials that transmit illumination of that particular wavelength. In a lithographic apparatus that applies that kind of illumination, like EUV radiation, the use of a reflective patterning structure is required.
Generally, a reflective patterning structure includes a substantially flat structure provided with a reflective surface. On the surface of the structure a radiation-absorbing layer is deposited and consecutively patterned. The radiation-absorbing layer, which typically has a thickness of about 50-500 nm, absorbs the illumination. The difference between the reflection coefficients of the reflective surface and the radiation-absorbing layer enables the transfer of the pattern from the patterning structure to the target portion on a substrate.
Examples of patterning structures include masks and programmable mirror arrays. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.
A support structure supports, i.e. bares the weight of, the patterning structure. It holds the patterning structure in a way depending on the orientation of the patterning structure, the design of the lithographic apparatus, and other conditions, such as, for example, whether or not the patterning structure is held in a vacuum environment. The support can use mechanical clamping, vacuum, or other clamping techniques, for example, electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning structure is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning structure”.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
In order to transfer the pattern of the patterning structure to the desired target portion on a substrate with extreme precision, the position of the patterning structure should be very well-defined. Before exposure, the patterning structure is placed on the support structure, for instance by using a robot arm. In one of several alignment steps, called pre-alignment, the position of the patterning structure with respect to the position of the support structure of the patterning structure is determined. Pre-alignment is most often carried out before the robot arm places the patterning structure on the support structure, because at that stage the position of patterning structure and support structure can be adjusted with respect to each other relatively easily.
For pre-alignment purposes, sensors are generally used to measure alignment markers at the system parts (patterning structure, substrate, support structure, stages etc.), which are aligned by illuminating them in reflection or transmission. In current optical lithography, a high contrast marker on a reflective patterning structure may be obtained by using the same techniques as used for patterning the patterning structure, i.e. by using an absorbing layer on top of a reflective substrate. However, different materials may be used to obtain the difference in reflectivity at these smaller wavelengths. As the markers are generally constructed adjacent to the pattern to be transferred, both elements may be composed of the same materials. As a result the illumination of the markers, used for pre-alignment purposes, should also include a beam having a smaller wavelength.
It is generally desirable to illuminate an alignment marker with a light source with a wavelength between 400-1500 μm. A light source for this range of wavelengths can easily be obtained and is generally inexpensive. Unfortunately, the contrast of the marker, originating from the difference in reflectivity of the materials that are used in a small-wavelength regime (like for instance EUV), deteriorates rapidly with larger wavelengths. At wavelengths between 400-1500 nm, the coefficients of reflectivity of the reflective surface and the absorbing layer used in small-wavelength lithography are about the same. Consequently, pre-alignment of a reflective patterning structure with respect to the support structure may be difficult to obtain on the basis of a difference of reflectivity between absorbing layer and reflective substrate using light at larger wavelengths.